Arion, VULGARIS

Reise, Heike, Schwarzer, Anne-Katrin, Hutchinson, John M. C. & Schlitt, Bettina, 2020, Genital morphology differentiates three subspecies of the terrestrial slug Arion ater (Linnæus, 1758) s. l. and reveals a continuum of intermediates with the invasive A. vulgaris Moquin-Tandon, 1855, Folia Malacologica 28 (1), pp. 1-34 : 16-17

ARION VULGARIS

( Figs 2 , 22–24 , 40–42 )

(1) A t r i u m a n d d i l a t e d o v i d u c t

( Figs 2 , 40 )

The upper, muscular atrium is smaller and not nearly as swollen and muscular as in A. ater s.l. Instead, the distal genitalia are dominated by a strong dilated oviduct. The thin oviduct connecting proximally to this muscular part is similar to that in A. ater s.l. (though usually shorter). Instead of the strong atrium retractor of A. rufus, a strong retractor inserts onto the distal part of the dilated oviduct.

(2) I n s e r t i o n o f d u c t s ( Figs 23 , 40 )

Epiphallus and bursa-duct enter the upper atrium proximally, inserting onto a lateral bulge of the atrium just beside the insertion of the dilated oviduct. In a specimen opened as described above, these insertions are thus visible, posterior to the oviduct insertion, without having to detach any genital retractor muscle (although in mature specimens, the bursa and/or the spermoviduct may have to be pushed aside). In contrast to the triangular arrangement in A. rufus, the insertions of oviduct, epiphallus, and bursa-duct lie roughly along a straight line (in that order).

(3) L i g u l a

( Figs 2 , 22–24 , 41 )

The ligula is in the dilated oviduct. If the slug has been opened as described above, and no genital retractor cut, before we open the oviduct the ligula base is on the wall of the oviduct lying upwards, away from the body wall. A cut along the oviduct in line with the retractor muscle allows one to turn over the ligula ( Figs 2 , 41 ), which should then be fully visible and undamaged.

The ligula is not bent and folded up as in A. ater s.l. but extended along the length of one side of the dilated oviduct; in comparison with A. ater s.l., one flank of the base is missing. The insertion of the thin oviduct is at the proximal end of the ligula base. The ligula takes the shape of a long boat with the ligula base corresponding to the bottom of the boat, the flange its sides. and the bow pointing towards the atrium ( DAVIES 1987). Our illustrations ( Figs 24 , 41 ) show the distal end of the oviduct orientated downwards ( Figs 24 , 41 ), so we will refer to the flange on the left side of the boat as the right flange (we imply no homology with the left and right flank in A. ater s.l.). The width of the flange is asymmetric, though much less so than in A. ater s.l., and it is also generally narrower and varies less between specimens. The right flange is thicker and stronger than the left but forms only a bulge rather than the prominent flap in A. ater s.l. At the proximal end (the stern of the boat), the flange running closely around the opening of the thin oviduct is very narrow. The opening of the thin oviduct is directly underneath its insertion: there is no tunnel running through the ligula base. The most prominent ligula structure is the bow of the boat, where both longitudinal folds meet distally and project as a thick tongue into the atrium ( Fig. 41 ); this tongue is attached only within the dilated oviduct. In a few individuals, a small single or double fold extends along the base, reminiscent of the two folds flanking the funnel-like structure on the right flank of the ligula in typical FR and BR.

(4) Posterior lip and

associated ridge

( Figs 22, 23 , 41 )

A ridge, generally less prominent in A. vulgaris than in A. ater s.l., leads to the opening of the bursa-duct, where it expands and forms a loop. This creates the spoon-shaped structure that ALLGAIER (2014) referred to as the posterior lip. The other end of the spoon continues as a small longitudinal fold towards the lower atrium.

(5) E p i p h a l l u s

( Fig. 42 )

The inner wall of the distal epiphallus contains longitudinal rows of big tubercles which are usually arranged in 5–6 well-ordered rows. Sometimes there are only 2 or 3 rows of big tubercles, and in this case there may be a few additional rows of smaller tubercles.

Figures

Figs 1–3. Schematic sketches of the genitalia of: 1 – Arion ater s.l., 2 – A. vulgaris (dashed lines in 1 and 2 indicate the cuts recommended), 3 – the opened upper atrium of A. ater s.l. viewed from the inside, with the dorsal side of the atrium in Fig. 1 reflected over to the right: the cut edge is indicated with a double line

Figs 5–24. Variability of the ligula within and between morphotypes of A. ater s.l. (5–10 – FR; 11–15 BR; 16–21 – AR) and A. vulgaris (22–24). Figures on the left show schematic views of the atrium, indicating the intra-morphotype range of configurations of the ligula base on the inside of the atrium: each variant configuration is distinguished by a different pattern of hatching or spotting. The ridge on the inside of the atrium is indicated with a dashed line and the site of attachment of the retractor muscle with horizontal hatching. Figures on the right show the range of shapes of the flattened-out ligula (viewed from the inside), distinguishing the base (attached to the atrium or oviduct wall) from the surrounding free-hanging flange. In the FR and BR morphotypes of A. ater s.l. a spongy funnel-shaped area of the ligula is shown; at the tip of the funnel is the oviduct opening, but this is often difficult to discern. In the AR morphotype the opening of the oviduct is usually instead surrounded by a circular swelling, and the opening is more apparent (Figs 19–21). Abbreviations: ovo – oviduct opening, lbrf – ligula base, right flank, lblf – ligula base, left flank

Figs 40–42. Genital anatomy of A. vulgaris (SMNG p17650, Rutschung P, Görlitz, DE, leg. 2005): 40 – the dissected genitalia closed back together to indicate the intact form; 41 – the atrium and distal oviduct opened to reveal the ligula (shaded olive: the left flank of the ligula is absent, and its flap is too small to show here); 42 – tubercles inside distal part of epiphallus. Abbreviations: bd – bursa-duct, ep – epiphallus, la – lower atrium, ua – upper atrium, frf – flap of right flank, rf – right flank of ligula, ov – oviduct, rt – retractor muscle